The present invention relates to an improved control unit for a bicycle of the type having rear mounted, leg actuated crank arms instead of the conventional rotary crank arms mounted on a sprocket between the front and rear wheels of standard bicycles. Specifically, the invention relates to the control of the torque applied to the rear wheel of such a bicycle.
Bicycles of this type have been known in the past and have been disclosed in certain foreign patents including Dutch Pat. No. 43,737 and French Pat. No. 432,826, and in an article in the October 1973 issue of Bicycling entitled "The Harris Vertical Bicycle".
The essence of this type of bicycle is that its rear wheel is driven alternately by a cable or chain moved back and forth by up and down movements of a pair of crank arms rotatably mounted on the bicycle frame behind the rear wheel. Sprockets on opposite sides of the hub of the rear wheel are coupled to the chain and are rotated thereby to drive the rear wheel in a forward direction. The sprockets are attached to a one-way clutch or ratchet mechanism so that each sprocket drives the rear wheel in the forward direction of rotation but is free-wheeling in the reverse direction. Moreover, when one crank arm is pulled downwardly the other crank arm is pulled upwardly by the action of the chain. Thus, the rider can drive the bicycle forwardly by continuous up and down leg movements with the feet on pedals at the front ends of the crank arms.
The prior art bicycles of this type generally provide a one-to-one relationship between the rotation of the two sprockets on the rear wheel hub and the up and down movements of the crank arms. This means that, for unit angular displacement of each crank arm, there will be a corresponding unit angular displacement of the corresponding sprocket for all operative positions of the crank arm. While this relationship is adequate for certain purposes, it is not especially desirable for other purposes, such as those cases where it is desired to obtain increased torque applied to the rear wheel for greater downward displacement of the crank arms. A need has, therefore, arisen for a control means for increasing this torque as a function of the displacement of the crank arms.